CI: run markdown linter on all pages + fixes

[chalin]

• Closes CI: check file formatting, including markdown #1063, for real
• Adds a Gulp task named lint-md, that runs markdownlint over all pages in a few seconds
• Fixes the reported errors in all pages
• Adds a GH action to run lint-md

Preview: This PR touches many files. Just poke around, in particular, have a look at:

• https://deploy-preview-3032--opentelemetry.netlify.app/docs/collector/benchmarks/

[chalin]

Best to review this file while ignoring whitespace in the diff. While the HTML was unindented, only a few other tweaks were done to this page.

[chalin]

From this log, you can see that the markdown check runs in 4.36 s (less than 30 for the entire job including installation of packages), https://github.com/open-telemetry/opentelemetry.io/actions/runs/5549104404/jobs/10132809311:

Run npm run check:markdown

> check:markdown
> npx gulp lint-md

[00:16:21] Using gulpfile ~/work/opentelemetry.io/opentelemetry.io/gulpfile.js
[00:16:21] Starting 'lint-md'...
Processed 293 files, 0 had issues.
[00:16:26] Finished 'lint-md' after 4.36 s

[chalin]

For the content files, spread across SIGs, only a few minor copyedits were done and/or a some markdownlint directives were added, so I'm going to merge now. Feedback, if any, I'll address in followup PRs.